1. Field of the Invention
The present invention relates to a serial axial fan.
2. Description of the Related Art
Electronic devices, such as personal computers (PCs) or servers, are commonly provided with a cooling fan for ventilation inside a case thereof and to cool electronic components contained therein. In particular, for use in comparatively large electronic devices such as servers, cooling fans that produce an air flow with high static pressure have been desired. One type of such cooling fans in current use are serial axial fans composed of two axial fans connected in series along a central axis such that impellers of the two axial fans rotate in opposite directions.
For example, in a serial axial fan described in JP-A-2008-95701, a vertical section of each blade of each impeller forms smaller angles with a rotational plane of the impeller at radially outward positions than at radially inward positions.
JP-A-2007-303432 discloses a counter-rotating blower in which a solidity ratio of a downstream axial fan in relation to an upstream axial fan is set in a range of 0.6 to 0.9. It is suggested that, according to this counter-rotating blower, setting a workload ratio of the downstream axial fan in relation to the upstream axial fan in a range of 0.6 to 0.9 will additionally reduce a decrease in air blowing efficiency.
Japanese Patent No. 4128194 discloses a counter-rotating axial blower in which an axial dimension of a first case provided in an inlet-side fan is larger than an axial dimension of a second case provided in an outlet-side fan.
JP-A 03-156193 discloses a counter-rotating ventilator in which a distance between a first impeller and a second impeller is set to 1.2 to 1.7 times an outer diameter of the impellers to achieve noise reduction.
The proportion of a workload of the impeller to a given power consumption is greatest at a radially outer portion of the impeller of axial fans. In the axial fan described in JP-A 2008-95701, the angle formed by each blade with the rotational plane of the impeller is arranged to be smaller at the radially outer portion thereof than at a radially inner portion thereof in order to increase air suction efficiency. This makes it difficult to generate most of work by the radially outer portion of the impeller.
A counter-rotating serial axial fan produces more noise than individual fans because of interference of blades of an inlet-side impeller and blades of an outlet-side impeller with each other. Accordingly, additional noise reduction is desired in accordance with an improvement in a “static pressure-air flow volume characteristic” of the serial axial fan.